The present invention relates to a solid-state image sensing device, and for example to a solid-state image sensing device having column ADCs (Analog to Digital Converters).
Many of solid-state image sensing devices for camera equipment or the like use CMOS sensors as image sensors and use simple integral ADC circuits for cost reduction. Further, even camera equipment or the like is required from the market to have a simple video shooting function. To increase the frame rate during video shooting without greatly increasing the overall cost, it is necessary to decrease the output bit precision of ADC to a level that does not greatly affect the image quality.
However, in such a CMOS sensor, for example, vertical line noise which does not appear in 12-bit output for photographs may appear in 10-bit output for video. This is known to be caused by quantization noise specific to a column ADC system having an ADC for each column of the image sensor. The following patent documents take the following measures against this problem.
In Japanese Unexamined Patent Publication No. 2008-60872 (Patent Document 1), a noise signal N2dim indicating random noise in two-dimensional space without temporal variation is added to a pixel signal So. A column ADC circuit (25) separately converts a reset level Srst and a signal level Ssig of the pixel signal having the noise signal N2dim added thereto into digital data independently, and takes the difference. By separately converting the two into digital data, with the noise signal N2dim added, the same noise effect appears on the AD conversion results of the two, thus canceling column-correlated noise. Although a quantization error associated with difference processing in a digital domain occurs, the quantization error can have no column correlation, and also the added noise signal N2dim can be removed. As for a specific way to generate the noise signal N2dim, a reset release interval TRelease is set to be shorter than a typical interval. That is, part of reset noise occurring during a reset operation by a reset transistor (36) as a pixel reset unit in a unit pixel (3) remains in the unit pixel as noise that is not removed by an offset removal operation by an operating point reset unit (offset removal unit) (330) in a voltage comparison unit (252).
In Japanese Unexamined Patent Publication No. 2011-50046 (Patent Document 2), a random offset of not more than +/−0.5LSB is added for each line to an offset of a DAC for generating a reference RAMP wave during ADC sampling, thereby reducing vertical streak noise. The offset addition is performed by current addition by an analog circuit.